Changes in State of Matter and Energy
Change of matter from one form to another involves energy changes. It can be done by boiling, evaporation, condensation, freezing, melting, sublimation and deposition.
Boiling point is the constant temperature at which liquid changes to vapour and the phenomenon is known as boiling. Boiling of water is a physical change that is desirable, reversible, fast and man-made.
Evaporation is the process by which a substance changes from the liquid state to the gaseous state at any temperature below its boiling point. The rate of evaporation depends upon temperature, moisture or humidity, surface area and movement of air.
Rate of evaporation increases with the increase in temperature and decreases with an increase in the moisture and humidity.
Rate of evaporation also increases with increase in surface area. Movement of air over the liquid also affects the rate of evaporation.
Condensation is the process by which a substance, changes from gaseous state to the liquid state on cooling. The condensation of water vapour to form clouds is a physical change that is desirable, reversible and natural.
Freezing is the process by which a substance changes from the liquid state to the solid state on cooling. The freezing of water into ice is a physical change that is desirable, reversible, fast and could be both natural and man-made.
Melting is the process by which a substance changes from the solid state to the liquid state on heating. The melting of ice cream is a physical change that is undesirable, reversible and fast.
Sublimation is a process by which a substance, changes from solid state to gaseous state directly without passing through the liquid state. The direct solidification of a vapour by cooling without going through the liquid state is called deposition. It is reverse of sublimation process.
Almost all chemical reactions are accompanied by energy changes.
An exothermic change is a change in which heat is given out to the surroundings.
In an exothermic change, the temperature of the reaction mixture rises.
An endothermic change is a change in which heat is absorbed from the surroundings. In an endothermic change, the temperature of the reaction mixture decreases.